(1) Field of the Invention
The present invention relates to a process for preparing a chemical composition, and more particularly, to a process for preparing sterically hindered hydroxyphenylcarboxylic acid esters, as well as to compositions containing the same.
(2) Description of Related Art
Many organic materials, such as polymers, fuels, and lubricants, are susceptible to oxidative and thermal deterioration from the action of heat, mechanical stress, and chemical reagents (such as atmospheric oxygen or metallic impurities). Deterioration of lubricants may result in an increase in their total acidity, formation of gums, discoloration, loss of physical properties such as viscosity, decreased potency, increasing molecular weight, polymerization, and the creation of odor. Changes in these properties can cause the lubricant to lose its effectiveness and longevity.
Oxidative degradation of a lubricant is a sequential process involving initiation, propagation, and termination phases. The initiation phase can begin by the formation of free radicals, which may be produced in a number of ways. For example, free radicals may be formed by reactive peroxides that are present during production of the lubricant, by thermal, mechanical or radiation stresses that occur during processing or end use, or by chemical reactions with impurities contained in the lubricant.
During the propagation phase, the free radicals can react with oxygen to form peroxy (RO2) and alkoxy (RO) radicals which, in turn, may abstract hydrogen from the lubricant to form unstable hydroperoxides (RO2H), alcohols (ROH), and new hydrocarbon free radicals (R). These free radicals can once again combine with oxygen to continue the oxidative cycle until the process slows or stops completely during the termination phase.
Lubricants are also subject to thermal degradation when used during periods of elevated temperatures or during periods of rapid cycling between elevated and low temperatures. Thermal degradation of a lubricant results in the disruption of long-chain hydrocarbons and causes the formation of unstable hydrocarbon compounds. These unstable compounds are especially prone to oxidation and can polymerize to form resins and sludge in the lubricant. For example, as an engine goes through multiple heating and cooling cycles, this sludge can harden and cause problems such as restricted passageways and decreased component tolerances.
One way to interrupt these destructive processes is to incorporate a stabilizer such as an antioxidant into the lubricant composition. Generally known antioxidants, such as hindered phenolic compounds, may be used to retard thermal and oxidative degradation. Such hindered phenolic compounds donate an active hydrogen atom to the oxidative free radicals formed during the initiation and propagation phases to ensure that the termination phase is reached quickly.
Unfortunately, while hindered phenol antioxidants are effective for combating the destructive effects of oxidation and thermal breakdown in lubricants, they are notoriously difficult to synthesize efficiently and in highly pure form. Conventional methods for the production of hindered phenol derivatives, particularly esters of phenols, often involve costly and time-consuming multi-step reaction procedures. These procedures usually require complicated isolation procedures for distilling the hindered phenol methyl ester intermediate and/or time-consuming and costly water washing steps to remove the unused or used catalysts (referred to herein as “catalyst residue”) during the production process.
For example, typical methods for preparing some hindered phenol antioxidants involve a Michael reaction between alkylphenols and an alkyl acrylate such as methyl acrylate, followed by extensive water washing and isolation of the resultant intermediate ester. Michael reactions are base-catalyzed conjugate additions of carbon nucleophiles (donors) to activated unsaturated compounds (acceptors). For the preparation of antioxidants, the Michael reaction donor is usually an alkylphenol compound and the acceptor is an unsaturated alkyl acrylate.
After formation of the intermediate ester through the Michael reaction, the intermediate ester is then subjected to a second step involving transesterification, followed once again by extensive washing and then purification of the solid antioxidant. Typically, the solid product is purified by crystallization and filtration.
In some methods, the separate transesterification step can be omitted if a suitable alcohol is incorporated into the alkylphenol/alkyl acrylate reaction. While such single-step reactions have advantages over multi-step preparation methods, such single-step reactions continue to have their disadvantages, particularly with respect to end-product purity.
To accelerate reaction rates, a base catalyst is usually added during both the Michael reaction and transesterification steps. Often, different catalysts at each step must be employed to achieve acceptable reaction times. Before the final antioxidant product can be isolated, however, all of the catalyst residue must be removed or the catalysts will contaminate the resulting antioxidant and, therefore, any lubricant made with the antioxidant.
Typically, the base catalysts are removed by first neutralizing them with acids such as acetic, hydrochloric or sulfuric. The antioxidant product is then precipitated away from the catalyst and filtration is used to separate the final product from the catalyst. However, catalysts are difficult to remove and cannot always be removed completely from the final product. In particular, while filtration may be suitable for the preparation of solid antioxidants, it is inappropriate and impractical for purifying liquid antioxidants.
One way to assist in the catalyst removal is through the use of extensive water washings of the reaction products at the end of each step. However, water washes slow down the overall time required to complete the antioxidant synthesis and add greatly to the inefficiency of these processes. Other reported methods to neutralize the catalyst generate large amounts of byproduct solids and lead to a waste disposal issue.
Improvements have been sought in many different reported procedures for producing a hindered phenolic compound. These procedures, however, do not adequately address all of the problems that arise from the making of hindered phenolic compounds.
For example, U.S. Pat. No. 4,085,132 to Park, et al. describes a one-step method of producing higher molecular weight hindered phenolic esters. According to this method, methyl acrylate is gradually added to a reaction mixture comprising 2,6-di-tert-butylphenol (which is a higher molecular weight monohydric hindered phenol), a high molecular weight alcohol, and a catalyst, without isolating the intermediate in a separate step. Higher excesses of methyl acrylate are required in order to drive this type of reaction to completion. The catalysts employed are alkaline metal catalysts of lithium, sodium and potassium. Catalyst neutralization is performed with acetic acid, which may not be effective for a water wash-free product isolation. The hindered phenol ester product is isolated as a solid by crystallization or recrystallization from the neutralized reaction mixture.
While such a method works well for solids, it is inappropriate and impractical for washing liquid hindered phenol products. Moreover, this type of isolation leads to significant yield loss of product in the filtrate and can result in high levels of impurities.
U.S. Pat. No. 4,228,297 to Haeberli, et al., on the other hand, describes one of the two-step methods where the Michael reaction is performed with one catalyst and the transesterification reaction is performed with a second catalyst that has a different chemical composition than the first catalyst. Again, the hindered phenol ester product is isolated as a solid by crystallization from the neutralized reaction mixture. All neutralizations are performed with acetic acid and all products are isolated by filtration. Again, this method is not practical for production and purification of liquid hindered phenol products.
Another example of a one-step reaction is set forth in U.S. Pat. No. 3,840,585 to Yamada, et al., wherein an alkyl acrylate is reacted with an alkylphenol to produce the final product. The patent describes the use of complex metal hydrides as catalysts. Such complex metal hydrides are very difficult to handle and to remove from the product but the process attempts to remove them by using acetic acid for neutralization. The reaction requires a promoter that is removed from the finished product by diluting the product with large volumes of toluene and then subsequently washing with water.
A method for the production of a hindered phenol methyl ester by the very rapid addition of methyl acrylate to the alkylphenol starting compound is described in U.S. Pat. No. 4,659,863 to Burton. However, hindered phenol alkyl ester products are not isolated in this patent. Acids that are suitable for catalyst neutralization according to this patent are acetic acid, hydrochloric acid and sulfuric acid.
U.S. Pat. No. 3,247,240 to Meier, et al., describes using alkali metal bases as catalysts in the Michael reaction. A variety of alkyl acrylates are used and all of the examples demonstrate higher alcohol products isolated by crystallization. The methyl ester is isolated by distillation and the reaction mixtures are neutralized with hydrochloric acid, followed by water washes. Distillation and crystallization processes are costly, time consuming, and lead to yield losses.
The one-step production method described in U.S. Pat. No. 3,364,250 to Dexter, et al., creates hindered phenolic compounds by substituting a higher alkyl acrylate for methyl acrylate. The catalyst is neutralized with hydrochloric acid and then removed through water wash steps and a distillation step. The hindered phenol product is isolated as a solid by crystallization from the neutralized reaction mixture.
Another method for producing a hindered phenol methyl ester is described in U.S. Pat. No. 3,330,859 to Dexter, et al. The two-step method involves purification by distillation through which the higher alkyl esters are crystallized as a solid and then catalyst neutralization is carried out with acetic and hydrochloric acid. This method is inappropriate for liquid transesterified hindered phenolics.
Finally, U.S. Pat. No. 6,559,105 to Abraham, et al., describes a Michael reaction that uses large amounts of magnesium silicate as an absorbent and filter aid to neutralize the potassium hydroxide catalyst. This process creates waste disposal issues due to the large volume of solids that are generated. For example, approximately 2.70% solids based on the weight of total phenolic ester product are produced.
There are also several process patents that describe specifically the transesterification step, including, for example, U.S. Pat. No. 6,291,703 to Schaerfl, et al., U.S. Pat. No. 4,694,099 to Ahlfors,  et al., U.S. Pat. No. 5,081,280 to Takee, et al., U.S. Pat. No. 5,136,082 to Dang, et al., U.S. Pat. No. 2,892,097 to Robertson, U.S. Pat. No. 4,594,444 to Orban, U.S. Pat. No. 4,536,593, and U.S. Pat. No. 4,716,244 to Orban.
The use of two different catalysts in such prior art processes is costly and ultimately requires the removal of both catalysts. Typically, these processes require repeated water washes to adequately remove the catalyst residue and purify the final product. Therefore, such processes are time-consuming.
While one might believe that time can be saved by employing one of the prior art single-step processes where the transesterification reaction and the Michael reaction are carried out in the same reaction mixture, these processes generally result in longer overall reaction times. Moreover, in these simultaneous, single-step, reactions, larger excesses of methyl acrylate are required to run the reaction and a lower purity product is generally obtained.
From the foregoing, it can be seen that a need exists for improved and more efficient methods for producing hindered phenols and, particularly, for producing sterically hindered phenol esters that can be used as antioxidants in compositions such as lubricants. It would also be useful to provide improved methods for producing hindered phenol antioxidants that do not rely on multiple catalyst additions and that do not require extensive water washing or difficult phase separation steps. Likewise, methods that offer improved processes for reducing the concentration of catalysts in the final hindered phenol product would also be desirable in that such products could be utilized as antioxidants for use in improved compositions such as better-formulated lubricants.